The SiC single crystal is a thermally and chemically stable compound semiconductor. The SiC single crystal has excellent physical properties compared to a Si single crystal. For example, the SiC single crystal has a larger band gap, higher insulation breakdown voltage, and higher thermal conductivity compared to a Si single crystal, and has a high saturation velocity of electrons. Therefore, the SiC single crystal is attracting attention as a next-generation semiconductor material.
The SiC exhibits crystal polytypism. Typical crystal structures of SiC are 6H, 4H, and 3C. Among these crystal structures, a SiC single crystal having a crystal structure of 4H has a larger band gap compared with a SiC single crystal having another crystal structure. Therefore, a SiC single crystal having a crystal structure of 4H is desired.
Known methods for producing a SiC single crystal include a sublimation recrystallization method and a solution growth method. In the sublimation recrystallization method, a raw material in a gas phase is fed onto a SiC seed crystal to grow a SiC single crystal.
In the solution growth method, a crystal growth surface of a SiC seed crystal is brought into contact with a Si—C solution and the Si—C solution in the vicinity of the SiC seed crystal is supercooled, and thereby a SiC single crystal grows on the SiC seed crystal. Here, the Si—C solution is a melt of Si or a Si alloy in which carbon (C) is dissolved. In a solution growth method, a graphite crucible is usually used as a container for accommodating the Si—C solution. In the graphite crucible, a raw material containing Si is melted by high-frequency induction heating, etc. to form a melt. In that case, C dissolves into the melt from the graphite crucible. As a result, the melt becomes a Si—C solution. The solution growth method is disclosed in, for example, Japanese Patent Application Publication No. 2009-91222 (Patent Literature 1).
In the solution growth method, melt-back is performed to remove strain and oxide film of the surface of the SiC seed crystal. The melt-back is a method in which the SiC seed crystal is brought into contact with the Si—C solution and then the temperature of the Si—C solution is increased. This causes the concentration of C in the Si—C solution to be unsaturated, thereby partially melting the crystal growth surface of the SiC seed crystal.
However, when the melt-back is performed, the Si—C solution may wet up on the side surface of the SiC seed crystal, or the side surface of a graphite support rod holding the SiC seed crystal. In this case, SiC polycrystals are likely to be generated.
International Application Publication No. 2012/127703 (Patent Literature 2) proposes a method for producing a SiC single crystal, which suppresses generation of SiC polycrystals. In Patent Literature 2, a wetting-up height of a solution onto a side surface of a SiC seed crystal is controlled to be within a range in which the SiC single crystal grown from the crystal growth surface and the SiC single crystal grown from the side surface grow as an integral SiC single crystal. Patent Literature 2 describes that, as a result of that, polycrystallization is suppressed.